1. Field of the Invention
The present invention generally relates to a method of measuring a flow rate of a fluid, such as liquid and gas and, in particular, to a method suitable for measuring a very small flow rate of a liquid and a flow meter for use in such method, as well as an apparatus for controlling a flow rate for use in a liquid using such flow meter.
2. Description of Related Art
Methods for measuring a very small flow rate of, for example, a liquid have included:
(1) a method in which a liquid is introduced into a vessel for an appointed time and a flow rate is determined on the basis of the weight of the liquid;
(2) a method in which a flow rate is determined on the basis of a time for filling up a liquid in a vessel having a known volume;
(3) a method using a rotor wheel and the like;
(4) a method in which a thermal sensor or a pressure sensor is disposed in a passage and a flow rate is determined from a velocity of flow; and
(5) a method in which a heater is wound around a tube, through which a liquid passes, and a flow rate is determined on the basis of a temperature of the heater itself, and the flow before and after the heater.
However, the above-described respective methods have the following disadvantages.
(1) and (2) cannot carry out the measurement in line nor the instantaneous flow rate. (3) is difficult to measure a very small flow rate due to the size of the rotor wheel and the like. (4) has problems in shielding a sensor portion and the material of a liquid junction portion. In addition, (5) has a problem in the sensitivity of measurement and generates bubbles when the liquid is heated by means of the heater and, furthermore, is unsuitable for the measurement of low-boiling point liquids.
The present invention has been achieved in view of the above-described matters, and it is an object of the present invention to provide a method, a flow meter, and an apparatus having a high accuracy of measurement from which the above-described disadvantages have sought to be eliminated.